Groin pain is a common complaint among distance runners, and they are at an increased risk of sports hernia for a few reasons.
It seems that a loss of hip extension range of motion forces an athlete to compensate with an external rotation pattern and going to the lower abdominal wall to “fake” extension and propel themselves well. This repetition thousands of time per run accelerates the loss of internal rotation range of motion and weakens the lower abdominal wall, which not only is implicated with the formation of “runners hernia” and labral tears, but also limits hip flexion and, over time, performance. Furthermore, repetitive, low-amplitude hip flexion over-recruits the TFL and rectus femoris, while leaving the psoas and iliacus underworked. When you call them into play with faster running, it is little doubt that they quickly tire and you run into “quad pulls” even runners hernia. Add to the mix the typical lack of core stability, strength, and an over-emphasis on long distance running and you have a recipe for hip injury.
Often, runners with groin pain begin to experience pain that occasionally radiates to the medial knee and also to the testicles. Much of the time this is a result of guarding. When you lose the ability to stabilize the pelvis and experience a hip injury your body will begin to “guard” the area by reducing mobility though muscle splinting. Essentially, your brain is telling the muscles around the injury to contract to provide stability to prevent the injured area from being disrupted. These muscles will quickly fatigue and form “trigger points” which are “knots” in muscle that send pain elsewhere. Several muscles, the adductors in particular, that are involved in guarding have trigger point referred pain patterns to the testicles (obviously one should take this seriously and have it investigated as such) and medial knee.
While in the long run it is obviously best to approach the issue multi-laterall (retraining movement patterns, soft tissue, modalities), people will often see a great deal of restoration of function and decrease in pain simply by seeing a good soft-tissue therapist who can help reduce tone in the adductors.